Dual impeller method and apparatus for effecting chemical conversion

ABSTRACT

Hydrogen sulfide or other gaseous component is removed from a gas stream containing the same by distribution of the gas stream in the form of fine bubbles by a rotary impeller and stationary shroud arrangement at a submerged location in an aqueous iron or other transition metal chelate solution, or other suitable catalyst, contained in an enclosed reaction vessel. Sulfur particles, or other insoluble phase product, of narrow particle size range formed in the reaction are floated off from the iron chelate solution. An oxygen-containing gas stream also is distributed in the form of fine bubbles by a separate rotary impeller and stationary shroud arrangement at a separate submerged location in the iron chelate solution. The second submerged location generally is separated from the first by a baffle extending downwardly in the reaction vessel from a top closure towards a bottom closure.

REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 709,158 filed Jun. 3, 1991,now abandoned which is a continuation-in-part of U.S. patent applicationSer. No. 446,777 filed Dec. 6,1989, now abandoned.

FIELD OF INVENTION

The present invention relates to method and apparatus for carrying outchemical reactions involving removal of gaseous components from gasstreams by chemical conversion to an insoluble phase while in contactwith a liquid phase.

BACKGROUND TO THE INVENTION

Many gas streams contain components which are undesirable and which needto be removed from the gas stream prior to its discharge to theatmosphere or further processing. One such component is hydrogensulfide, while another such component is sulfur dioxide.

The combustion of sulfur-containing carbonaceous fuels, such as fueloil, fuel gas, petroleum coke and coal, as well as other processes,produces an effluent gas stream containing sulfur dioxide. The dischargeof such sulfur dioxide-containing gas streams to the atmosphere has leadto the incidence of the phenomenon of "acid rain", which is harmful to avariety of vegetation and other life forms. Various proposals have beenmade to decrease such emissions.

Hydrogen sulfide occurs in varying quantities in a variety of gasstreams, for example, in sour natural gas streams and in tail gasstreams from various industrial operations. Hydrogen sulfide isodoriferous, highly toxic and a catalyst poison for many reactions andhence it is desirable and often necessary to remove hydrogen sulfidefrom such gas streams.

There exist several commercial processes for effecting hydrogen sulfideremoval. These include processes such as absorption in solvents, inwhich the hydrogen sulfide first is removed as such and then convertedinto elemental sulfur in a second distinct step, such as a Claus plant.Such commercial processes also include liquid phase oxidation processes,such as Stretford, LO-CAT, Unisulf, Sulferox, Hiperion and others,whereby the hydrogen sulfide removal and conversion to elemental sulfurnormally are effected in reaction and regeneration steps.

In Canadian Patent No. 1,212,819 and its corresponding U.S. Pat. No.4,919,914, the disclosure of which is incorporated herein by reference,there is described a process for the removal of hydrogen sulfide fromgas streams by oxidation of the hydrogen sulfide at a submerged locationin an agitated flotation cell in intimate contact with an iron chelatesolution and flotation of sulfur particles produced in the oxidationfrom the iron chelate solution by hydrogen sulfide-depleted gas bubbles.In this prior art operation, both an oxygen-containing gas stream and ahydrogen sulfide-containing gas stream are distributed as fine gaseousbubbles at the same submerged location in the iron chelate solution toeffect oxidation of the hydrogen sulfide.

In practice, it has been found that the quantity of oxygen required tobe provided to effect substantially complete oxidation of the hydrogensulfide to sulfur significantly exceeds the stoichiometric quantitytheoretically required and experimentation has been unable to decreasethe oxygen requirement below about five times stoichiometric. In otherprior art hydrogen sulfide-removal processes, generally more than twentytimes the stoichiometric quantity of oxygen is required.

SUMMARY OF INVENTION

In accordance with the present invention, there is provided a novelprocedure for carrying out the hydrogen sulfide removal process outlinedabove whereby the oxygen usage is significantly improved, as well asnovel equipment for carrying out such procedure.

The present invention is particularly concerned with the removal ofhydrogen sulfide from a gas stream containing the same by a novelprocedure and to novel equipment for effecting the same. However, theprinciples of the present invention are generally applicable to theremoval of gas, liquid and/or solid components from a gas stream bychemical reaction. In the present invention, an efficient contact of gasand liquid is carried out for the purpose of effecting a reaction whichremoves a component of the gas and converts that component to aninsoluble phase while in contact with the liquid phase.

There are a variety of processes to which the principles of the presentinvention can be applied. The processes generally involve reaction ofthe component with another gaseous species in a liquid phase, usually anaqueous phase, often an aqueous catalyst system.

One example of such a process is the oxidative removal of hydrogensulfide from gas streams in contact with an aqueous transition metalchelate system to form sulfur particles, as described generally in theabove-mentioned Canadian Patent No. 1,212,819.

Another example of such a process is in the oxidative removal ofmercaptans from gas streams in contact with a suitable aqueous catalystsystem to form immiscible liquid disulfides.

A further example of such a process is the oxidative removal of hydrogensulfide from gas streams using chlorine in contact with an aqueoussodium hydroxide solution, to form sodium sulphate, which, after firstsaturating the solution, precipitates from the aqueous solution.

An additional example of such a process is the removal of sulfur dioxidefrom gas streams by the so-called "Wackenroder's" reaction by contactinghydrogen sulfide with an aqueous phase in which the sulfur dioxide isinitially absorbed, to form sulfur particles. This process is describedin U.S. Pat. Nos. 3,911,093 and 4,442,083. The procedure of the presentinvention also may be employed to effect the removal of sulfur dioxidefrom a gas stream into an absorbing medium in an additional gas-liquidcontact vessel.

A further example of such a process is the removal of sulfur dioxidefrom gas streams by reaction with an aqueous alkaline material.

The term "insoluble phase" as used herein, therefore, encompasses asolid insoluble phase, an immiscible liquid phase and a component whichbecomes insoluble when reaching its solubility limit in the liquidmedium after start up.

In one aspect of the present invention, there is provided a method forthe removal of a gaseous component from a gas stream containing the sameby chemical conversion of the gaseous component to an insoluble phase ina liquid phase, comprising a plurality of steps.

In this method, an enclosed reaction zone is provided having a liquidmedium. The gaseous component-containing gas stream is fed to a firstsubmerged location in the liquid medium and is distributed at the firstsubmerged location in the form of small gas bubbles. A second gasstream, different from the gaseous component-containing stream, is fedto a second submerged location in the liquid medium spaced apart fromthe first submerged location therein and is distributed at the secondsubmerged location in the form of small gas bubbles.

Interaction is permitted between the small gas bubbles of the gaseouscomponent-containing gas stream, the small gas bubbles of the second gasstream and the liquid medium to effect conversion of the gaseouscomponent to the insoluble phase in the liquid medium.

The reaction zone preferably is provided with divider means thereinextending from a top closure to the reaction zone downwardly into theliquid medium to establish first and second individual reaction zonesseparated physically one from another by the divider means but in liquidflow communication with each other.

In the embodiment employing the divider means, the first submergedlocation is located in the first individual reaction zone and the secondsubmerged location is located in the second individual reaction zone.

Any desired gas-liquid contact means may be employed at each of thesubmerged locations in order to effect distribution of the gas stream assmall gas bubbles at the respective submerged location. Preferably, thegas-liquid contact means at each submerged location comprises a shroudand impeller combination as described in more detail below.

The insoluble phase formed in the liquid medium often is provided in aform which is flotable by the gas bubbles after the interaction. In apreferred embodiment, the depleted gas bubbles are permitted to risethrough the liquid medium in the respective individual reaction zonesand to float the insoluble phase on the surface of the liquid medium inthe respective individual reaction zones. However, where appropriate,the solid phase may be filtered off.

For the removal of hydrogen sulfide from a gas stream by oxidation tosulfur, using an oxygen-containing gas stream as the second gas stream,an aqueous transition metal catalyst solution is employed as the liquidmedium. By introducing the oxygen-containing gas stream at a differentsubmerged location from the hydrogen sulfide-containing gas stream, ithas been found that the quantity of oxygen required for oxidativeremoval of hydrogen sulfide can be considerably decreased compared tothe process of Canadian Patent No. 1,212,819, to less than two timesstoichiometric. In addition, by introducing the gas streams at differentlocations within the aqueous phase and by employing a baffle separatingthe reaction zones, any danger of forming an explosive gas mixture ofthe oxygen-containing gas stream and the hydrogen sulfide-containing gasstream is eliminated.

The present invention also includes novel apparatus for effectinggas-liquid contact reactions, including the removal of hydrogen sulfidefrom gas streams. In accordance with a second aspect of the invention,such apparatus includes a plurality of elements. An enclosed vessel hasdivider means extending downwardly within the vessel from an upperclosure thereof towards a lower closure to divide the vessel into firstand second separate reaction zones which are in liquid flowcommunication one with another via the body of liquid medium.

First gas feed pipe means extends downwardly in one of the reactionzones. First rotary impeller means is located at the lower end of thefirst gas pipe means and is mounted for rotation about a vertical axis.First shroud means surrounds the first rotary impeller means and has aplurality of openings therethrough.

Second gas feed pipe means extends downwardly in the other of thereaction zones. Second rotary impeller means is located at the lower endof the second gas feed pipe means and is mounted for rotation about avertical axis. Second shroud means surrounds the second rotary impellermeans and has a plurality of openings therethrough.

In the apparatus, therefore, two individual combinations of gas feedpipe, impeller and shroud are provided in separate reaction zonesphysically separated by a divider or baffle.

While the present invention is directed particularly to the removal ofhydrogen sulfide from gas streams containing the same and will bedescribed in particular with reference thereto, the invention is morebroadly based, as described above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of an apparatus constructed inaccordance with one embodiment of the invention; and

FIG. 2 is a schematic representation of an apparatus constructed withanother embodiment of the invention.

GENERAL DESCRIPTION OF INVENTION

One embodiment of the present invention is directed towards removinghydrogen sulfide from gas streams. High levels of hydrogen sulfideremoval efficiency are attained, generally in excess of 99.99%, from gasstreams containing any concentration of hydrogen sulfide. Residualconcentrations of hydrogen sulfide less than 0.1 ppm by volume can beattained.

The process of the invention is able to remove effectively hydrogensulfide from a variety of different source gas streams containing thesame, provided there is sufficient oxygen to oxidize the hydrogensulfide. The oxygen may be present in the hydrogen sulfide-containinggas stream to be treated or may be separately fed, as is desirable wherenatural gas or other combustible gas streams are treated.

The process of the present invention is able to remove effectivelyhydrogen sulfide from a variety of different source gas streamscontaining the same, provided there is sufficient oxygen to oxidize thehydrogen sulfide. Such gas streams include fuel gas and natural gas andother hydrogen sulfide-containing streams, such as those formed in oilprocessing, in oil refineries, mineral wool plants, kraft pulp mills,rayon manufacturing, heavy oil and tar sands processing, coking coalprocessing, meat rendering, a foul gas stream produced in themanufacture of carborundum, and gas streams formed by air strippinghydrogen sulfide from aqueous phases. The gas stream may be onecontaining solid particulates or may be one from which such particulatesare absent. The ability to handle a particulate-laden gas stream withoutplugging may be beneficial, since the necessity for upstream cleaning ofthe gas is obviated.

The process of the present invention for effecting removal of hydrogensulfide from a gas stream containing the same employs a transition metalchelate in aqueous medium as the catalyst for the oxidation of hydrogensulfide to sulfur. The transition metal usually is iron, although othertransition metals, such as vanadium, chromium, manganese, nickel andcobalt may be employed. Any desired chelating agent may be used butgenerally, the chelating agent is ethylenediaminetetraacetic acid(EDTA). An alternative chelating agent is HEDTA. The transition metalchelate catalyst may be employed in hydrogen or salt form. The operativerange of pH for the process generally is about 7 to about 11.

The hydrogen sulfide removal process is conveniently carried out atambient temperatures of about 20° to 25° C., although higher and lowertemperatures may be utilized and still achieve an efficient operation.The temperature is generally ranges from about 5° to about 80° C.

The minimum catalyst concentration to hydrogen sulfide concentrationratio for a given gas throughput may be determined from the rates of thevarious reactions occurring in the process and is influenced by thetemperature and the degree of agitation or turbulence in the reactionvessel. This minimum value may be determined for a given set ofoperating conditions by decreasing the catalyst concentration until theremoval efficiency with respect to hydrogen sulfide begins to dropsharply. Any concentration of catalyst above this minimum may be used,up to the catalyst loading limit of the system.

The process of the present invention generally is effected in a uniqueapparatus, which constitutes one aspect of the present invention. Theapparatus comprises an enclosed vessel containing a body of the chelatediron catalyst solution and the catalyst solution preferably is dividedinto two zones by an internal divider or baffle extending downwardlyfrom an upper closure to the vessel into the catalyst solution to aportion of the depth thereof. The purpose of the divider or baffle, whenpresent, is to prevent mixing of the gases in the gas spaces above therespective reaction zones in the catalyst solution, and to provide tworeaction zones in the catalyst solution which are physically separatefrom each other.

The baffle extends only part-way downwardly within the body of catalystsolution, so that there is common body of catalyst solution below thelower edge of the baffle. The baffle may be constructed of anyconvenient material of construction which achieves this result. Thebaffle may be constructed of a solid material, or, alternatively, in theportion immersed in the liquid phase, the baffle may be in the form of afine mesh material which permits liquid flow therethrough but whichinhibits the flow of the small gas bubbles therethrough. The mesh may berendered water-wettable to cause the gas to coalesce.

By providing separate reaction zones within the body of catalystsolution, mixing of the gas streams is largely prevented. Although someflow of hydrogen sulfide-containing gas to the reaction zone into whichthe oxygen-containing gas stream is fed can be tolerated, it is highlyundesirable for the oxygen-containing gas stream to flow to the reactionzone into which the hydrogen sulfide-containing stream is fed, sincethis flow may lead to contamination of the product gas stream from thehydrogen sulfide removal process, which would be highly undesirable withcertain gas streams, for example, natural gas streams. Another advantageis that, if further treatment of that gas stream is required, a lesservolume results than if the gases are mixed.

Although the invention is described particularly with respect to theprovision of two separate reaction zones within the body of catalystsolution, it will be readily apparent to those skilled in the art thatmore than two reaction zones may be employed, as desired, by employingadditional baffles downwardly-extending into the catalyst solution. Inaddition, it is also possible to place more than one impeller in one ofthe reaction zones. As noted above, baffled-physical separation of thereaction zones also may be omitted, if desired.

The removal of hydrogen sulfide by the process of the present inventionis carried out in an enclosed gas-liquid contact zone in which islocated an aqueous medium containing transition metal chelate catalyst.A hydrogen sulfide-containing gas stream and an oxygen-containing gasstream, which usually is air but may be oxygen or oxygen-enriched air,are caused to flow along a vertical flow path from outside thegas-liquid contact zone to separate submerged locations in the aqueouscatalyst medium from which the gas streams are forced by a rotatingimpeller in each reaction zone to flow through openings in a shroudsurrounding the impeller into the body of the aqueous medium. Eachimpeller comprises a plurality of outwardly-extending blades and isrotated about a generally vertical axis. The rotating impeller alsodraws the liquid phase to the locations of introduction of the gasstreams from the body of aqueous medium in the enclosed zone. When avertical baffle is employed, a standpipe may pass upwardly to theimpeller and a guide pipe may extend from the baffle to the respectiveimpeller to assist in aqueous medium being drawn from one reaction zoneto the impeller in the other to enhance the respective reactionsthereat.

The gas-induction impeller and accompanying shroud may be constructed inthe manner conventionally employed in an agitated flotation cell, asdescribed in the aforementioned Canadian Patent No. 1,212,819.Alternatively, and preferably, the combination may be provided in themanner described in copending U.S. patent application Ser. No. 622,485filed Dec. 5, 1990, now U.S. Pat. No. 5,174,973 (continuation-in-part ofU.S. patent application Ser. No. 582,423 filed Sep. 14, 1990, nowabandoned itself a continuation-in-part of U.S. patent application Ser.No. 446,776 filed Dec. 6, 1989, now abandoned), in which one of us(James W. Smith) is named as an inventor, the disclosures of which areincorporated herein by reference. A variety of relative parameters andstructural modifications are described in these latter patentapplications, which also apply to the illustrated embodiments ofapparatus described herein below, and are expressly incorporated hereinby reference.

As noted earlier, the impeller-shroud combination is one embodiment ofgas-liquid contact means which may be employed to achieve distributionof the gases as fine bubbles in the reaction medium.

If desired, circulation of aqueous medium within individual reactionzones or within the enclosed vessel may be effected by the employment ofsuitable pumping mechanisms.

The distribution of the gases as fine bubbles in the reaction medium inthe region of the impellers enables a high rate of mass transfer tooccur. In the body of catalyst solution, a complicated series ofchemical reactions occurs resulting in an overall reaction which isrepresented by the equation:

    H.sub.2 S+1/2O.sub.2 →S+H.sub.2 O

This overall reaction results in depletion of hydrogen sulfide from thehydrogen sulfide-containing gas stream to effect substantial removaltherefrom and depletion of oxygen from the oxygen-containing gas stream.

The solid sulfur particles grow in size until of a size which can befloated. Alternative procedures of increasing the particle size may beemployed, including spherical agglomeration or flocculation. Theflotable sulfur particles are floated by the oxygen-depleted gas bubblesrising through the body of catalyst solution and collected as a froth onthe surface of the aqueous medium. The sulfur particles range in sizefrom about 10 to about 50 microns in diameter and are in crystallineform.

The hydrogen sulfide-depleted gas stream is removed from a gas spaceabove the liquid level in the reaction zone to which the hydrogensulfide-containing gas stream is fed. Since this gas space is physicallyseparated from a similar gas space above the liquid level in thereaction zone to which the oxygen-containing gas stream is fed when thebaffle is employed, the product gas stream is uncontaminated by oxygen.

The series of reactions which is considered to occur in the body of themetal chelate solution to achieve the overall reaction noted above is asfollows:

    H.sub.2 S=H.sup.+ +HS.sup.-

    OH.sup.- +FeEDTA.sup.- =[Fe.OH.EDTA].sup.=

    HS.sup.- +[Fe.OH.EDTA].sup.= =[Fe.HS.EDTA].sup.= +OH.sup.-

    [Fe.HS.EDTA].sup.= =FeEDTA.sup.- +S+H.sup.+ +2e

    2e+1/2O.sub.2 +H.sub.2 O=2OH.sup.-

    2OH.sup.- +2H.sup.+ =2H.sub.2 O

As may be seen from these equations, the stoichiometric use of oxygenfor the oxidation of hydrogen sulfide to sulfur requires one-half moleof oxygen for each mole of hydrogen sulfide. As noted earlier, mostprior art hydrogen sulfide-removal procedures involving oxidation ofhydrogen sulfide employ large excesses of oxygen with respect tostoichiometric. By the present invention, the oxygen requirement hasbeen considerably decreased while obtaining highly-efficient removal ofhydrogen sulfide from a variety of gas streams and, at the same time,providing the ability to avoid contamination of the product gas streamwith oxygen. In experimentation, it has been found possible to decreaseoxygen usage to below two times stoichiometric, generally toapproximately 50% greater than stoichiometric.

Another embodiment of the invention is directed towards removing sulfurdioxide from gas streams. The procedure for such removal shows manysimilarities with the hydrogen sulfide-removal procedure just described,except that the aqueous medium in this case contains an alkalinematerial.

The aqueous alkaline medium into which the sulfur dioxide-containing gasstream is introduced may be provided by any convenient alkaline materialin aqueous dissolution or suspension. One convenient alkaline materialwhich can be used is an alkali metal hydroxide, usually sodiumhydroxide. Another convenient material is an alkaline earth metalhydroxide, usually a lime slurry or a limestone slurry.

Absorption of sulfur dioxide in an aqueous alkaline medium tends toproduce the corresponding sulfite. It is preferred, however, that thereaction product be the corresponding sulfate, in view of the greatereconomic attraction of the sulfate salts. For example, where lime orlimestone slurry is used, the by-product is calcium sulfate (gypsum), amulti-use chemical.

Accordingly, in a preferred aspect of the invention, anoxygen-containing gas stream, which usually is air but which may be pureoxygen or oxygen-enriched air, analogously to the case of hydrogensulfide, is separately introduced to the aqueous alkaline reactionmedium at a submerged location physically separate from that at whichthe SO₂ -containing gas stream is introduced, so as to cause the sulfatesalt to be formed. When such oxidation reaction is effected in thepresence of a lime or limestone slurry, it is generally preferred to adda small amount of an anti-caking agent, to prevent caking of theby-product calcium sulfate on the lime or limestone particles,decreasing their effectiveness. One suitable anti-caking agent ismagnesium sulfate.

The concentration of sulfate salt builds up in the aqueous solutionafter initial start up until it saturates the solution, whereupon thesulfate commences to precipitate from the solution. The crystallinesulfate, usually sodium sulfate or calcium sulfate crystals, may befloated from the solution by the oxygen-depleted gas bubbles, ifdesired, with the aid of flotation-enhancing chemicals, if required.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, there is illustrated in FIG. 1 an enclosedapparatus 10 for effecting removal of hydrogen sulfide from a hydrogensulfide-containing gas stream in accordance with one preferredembodiment of the invention. The apparatus 10 contains a body of aqueousiron chelate solution 12, or other convenient transition metal chelatesolution, and an internal baffle 14 extending downwardly into theaqueous iron chelate solution 12 from an upper closure 15 towards butspaced from a lower closure 16, dividing the vessel 10 into two reactionzones 8 and 9.

A gas feed pipe 17, 18 extends downwardly into the apparatus 10 on eachside of the baffle 14 into the respective reaction zones 8 and 9. Animpeller 20, 22 is provided adjacent to the lower end of the gas feedpipe 17, 18 respectively and is mounted for rotation about an axle 24,26, so as to induce gas flow into and through the pipes 17, 18. Ifdesired, a fan or blower may be used to increase the gas flow rate toone or both of the impellers 20, 22. Each impeller 20, 22 comprises aplurality of radially-extending blades. In the case of pipe 17, ahydrogen sulfide-containing gas stream 28, such as, a sour natural gasstream, is induced and, in the case of pipe 18, an oxygen-containing gasstream 30, such as air, oxygen or oxygen-enriched air, is induced.

Surrounding each impeller 20, 22 is a cylindrical stationary shroud 32,34, which has a plurality of openings 36 therethrough, which, combinedwith the rotation of the impeller 20, 22, results in dispersion of thegases fed through the respective pipes 17 and 18 in the form of finebubbles. Dispersion of the fine bubbles of hydrogen sulfide-containinggas stream in the iron chelate solution promotes gas-liquid contact andrapid reaction of the hydrogen sulfide to sulfur in the iron chelatesolution. Although, in the illustrated embodiment, the shrouds 32 and 34are right cylindrical and stationary, it is possible for one or both ofthe shrouds 32 and 34 to possess other shapes. For example, the shrouds32, 34 may be tapered, with each impeller 20, 22 optionally beingtapered. In addition, one or both of the shrouds 32, 34 may be rotated,if desired, usually in the opposite direction to the respective impeller20, 22.

Dispersion of the fine bubbles of oxygen-containing gas stream in theiron chelate solution promotes gas-liquid contact and rapid regenerationof the iron chelate solution. The various reactions which occur in thebody of iron chelate solution 12 are described above and result in anoverall reaction in the reactor 10 represented by the equation:

    H.sub.2 S+1/2O.sub.2 →S+H.sub.2 O

The hydrogen sulfide is removed from the hydrogen sulfide-containing gasstream in contact with the iron chelate solution and bubbles of hydrogensulfide-depleted gas rise in the reaction zone 8 towards the surface ofthe iron chelate solution in that zone. Similarly, oxygen is removedfrom the oxygen-containing gas stream in contact with the iron chelatesolution and bubbles of oxygen-depleted gas rise in the reaction zone 9towards the surface of the iron chelate solution in that zone.

The fine sulfur particles which are formed grow in the body of the ironchelate solution until they reach a size which permits them to befloated to the surface of the iron chelate solution in the respectivereaction zones 8 and 9 by the respective bubbles of depleted gas stream,to form a sulfur froth 38 on the iron chelate solution surface. Thesulfur is obtained in orthorhombic crystalline form with a particle sizeranging from about 10 to about 30 microns. This narrow particle sizerange permits ready separation of the sulfur from entrained iron chelatesolution in further processing of the froth 38. The sulfur may beremoved from the surface of the iron chelate solution in each of thezones 8 and 9 by respective skimmers 40, 42 into launders 44, 46.

The hydrogen sulfide-depleted gas stream is collected in a gas space 48above the surface of the iron chelate solution in zone 8 and is removedby line 50.

The oxygen-depleted gas stream is collected in the gas space 52 abovethe surface of the iron chelate solution in zone 9 and is removed byline 54. The presence of the baffle 14 ensures that the gas spaces 48and 52 are physically separated one from another, so that the respectivedepleted gas streams cannot mix.

Similarly, the presence of the baffle 14 extending downwardly into thebody 12 of iron chelate prevents the oxygen-containing gas stream fed tothe reaction zone 9 from entering the reaction zone 8, so thatcontamination of the product gas stream in line 50 by oxygen is avoided.

In the illustrated embodiment, the impeller and shroud combination 22and 34 for the oxygen-containing gas stream is smaller than the impellerand shroud combination 20 and 32 for the hydrogen sulfide-containing gasstream. This arrangement is the usual one, since the concentration ofhydrogen sulfide in the gas stream being treated is usually very muchless than the concentration of oxygen in the oxygen-containing gasstream. However, the impeller-shroud combinations may have the same orlarger size, as desired.

FIG. 2 differs from FIG. 1 in that the outlet 54 for oxygen-depleted gasstream is fed to the inlet pipe 17 for the hydrogen sulfide-containinggas stream. As a result, oxygen present in the oxygen-depleted gasstream is distributed along with the hydrogen sulfide in reaction zone 8and is rapidly consumed therein, thereby further decreasing the overalloxygen requirement to close to stoichiometric. This arrangement is alsobeneficial where some hydrogen sulfide-depleted gas bubbles have enteredzone 9 and hence are collected along with the oxygen-depleted gas in thegas space 52.

By providing separate feeds of hydrogen sulfide-containing gas streamand oxygen-containing gas stream into two separate reaction zones withinthe same body of iron chelate catalyst solution, in contrast to thearrangement described in Canadian Patent No. 1,212,819, where both gasstreams are fed to the same submerged location in the iron chelatesolution, a considerably-improved process efficiency, in terms of oxygenusage, is obtainable. As mentioned above, the best result obtainablewith the prior system required five times stoichiometric use of oxygen,whereas by using the arrangement illustrated in FIG. 1 of the drawings,less than twice the stoichiometric amount of oxygen is required.

One particular advantage that the present invention provides is withrespect to the processing of natural gas and similar flammable gasfeeds. Since the oxygen-containing gas stream does not come into contactwith the hydrogen-sulfide gas stream during the hydrogen sulfide removaloperation, potentially explosive gas mixtures are not formed. Inaddition, if further processing of the gas stream is required, thevolume of gas to be handled has not been increased.

In addition, since the gases are separately fed to separate submergedlocations, there is no mutual dilution of the hydrogen sulfide andoxygen in the respective gas streams fed to the reactor 10, so thatthere is achieved a much higher mass transfer rate at each impeller 20,22 than is achieved in Canadian Patent No. 1,212,819. In the latterpatent, the gas streams both are fed to the same submerged location,either as a mixture of gases or separately, so that the gases mutuallydilute each other at the submerged locations. As a result of the highermass transfer rate achieved herein, higher concentrations of hydrogensulfide can be treated in the same size of equipment.

In addition to the above-noted advantages, the present invention alsoshares the advantages of the system described in Canadian Patent No.1,212,819, namely that hydrogen sulfide is rapidly and efficientlyremoved from gas streams containing the same, the by-product sulfur isobtained in a narrow particle size range, and induction of the gases iseffected at low pressure drop, thereby decreasing the need for pumping.

As mentioned above, the present invention is not limited to thetreatment of hydrogen sulfide-containing gas streams to remove thehydrogen sulfide therefrom but is broadly directed to any process inwhich a gaseous component is reacted to an insoluble phase in a liquidmedium, often in a form which then can be floated from the solution.Alternatively, other removal methods, such as filtration, may beemployed.

For example, the apparatus 10 illustrated in FIGS. 1 and 2 may beemployed to effect the removal of mercaptans from a gas streamcontaining the same, which is fed by line 28. In this process, the metalchelate solution 12 is replaced by an aqueous sodium hydroxide solutionand the liquid disulfides which result from the oxidation are floatedoff and removed from the surface of the sodium hydroxide solution.

Gas streams contaminated with hydrogen sulfide often also arecontaminated by mercaptans, such as sour natural gas streams. Inaccordance with one embodiment of the invention, both components may beremoved from a gas stream containing them by a sequential operation inwhich the hydrogen sulfide or mercaptans first is removed from the gasstream in a first reactor 10 and the other gas subsequently is removedfrom the gas stream by feeding the product gas stream from the firstreactor 10 to a second reactor 10.

EXAMPLE

An experimental apparatus was constructed in accordance with FIG. 1 andexperiments were conducted in the apparatus to determine the minimumamount of oxygen required by the two-impeller system, with a hydrogensulfide-containing gas stream being fed to one impeller and with oxygenonly being fed to the other impeller.

On the H₂ S-impeller side, known volumes of hydrogen sulfide wereintroduced into a nitrogen-bearing gas stream while in the secondchamber, a known amount of oxygen was introduced. Above the liquidlevel, a gas tight barrier was provided while below the liquid level, afine mesh was provided which allowed a portion of the liquid to passthrough while excluding all but the finest of bubbles.

Initially the system was caused to sulfide by flowing excess amounts ofhydrogen sulfide and no oxygen into the reactor, which contained a bodyof iron chelate solution. Sulfiding was characterized by the formationof a black-olive solution, as opposed to the normal pale browncoloration, and very poor H₂ S removal.

The oxygen flow rate was slowly increased and the H₂ S outletconcentration measured continuously. A point was reached where theremoval rate of hydrogen sulfide started to increase, as hydrogensulfide outlet concentration fell; which was the point where there wasjust enough oxygen to regenerate sufficient catalyst to replace thatsulfide by the H₂ S. The value then was the minimum oxygen requirementto maintain the reactor system. The procedure was performed at differentgas flow rates and rpm.

The results obtained are tabulated in Tables I, II and III below:

                  TABLE I                                                         ______________________________________                                        Operating Conditions                                                          ______________________________________                                        Inlet hydrogen sulfide concentration                                                                  1000   ppm                                            Sodium ion concentration                                                                              0.02   molar                                          Iron concentration      1      g/L                                            Operating pH range      9.0 to 9.2                                            ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Oxygen Flow Rate (mL/min)                                                             Nitrogen Flow Rate (L/min)                                            RPM       10      20          30    50                                        ______________________________________                                         600      15.4    42.7        73.5                                             900      8.08    31.9                                                        1200      7.64    21.1        55.1  125                                       1500              16.4        42.1                                            1800      7.02    15.4        38.5  106                                       ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Mol Ratio of Oxygen to H.sub.2 S                                              Nitrogen Flow Rate (L/min)                                                    RPM        10      20         30    50                                        ______________________________________                                         600       1.5     2.1        2.4                                              900       0.81    1.9                                                        1200       0.76    1.1        1.8   2.5                                       1500               0.82       1.4                                             1800       0.70    0.77       1.3   2.1                                       ______________________________________                                    

Table III shows the mole ratio of oxygen required to H₂ S consumed.Theoretically, 0.5 mol of O₂ is required per mol of H₂ S. The minimumoxygen required is 1.4 times Stoichiometric, as shown in Table III at 10L/min N₂ and 1800 rpm.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides a novelmethod and apparatus for the efficient removal of gaseous componentsfrom gas streams, employing a dual-impeller arrangement for the separatedistribution of a gaseous component-containing gas stream and a secondgas stream as fine bubbles in a suitable liquid medium for formation ofan insoluble phase, which can be collected by flotation, if desired.Modifications are possible within the scope of this invention.

What we claim is:
 1. An apparatus for effecting gas-liquid contactreactions, which comprises:an enclosed vessel for holding a body ofliquid medium in which to effect gas-liquid contact reactions; bafflemeans extending downwardly within said vessel from an upper closurethereof towards a lower closure thereof terminating in a lower extremityspaced from said lower closure to divide said vessel into a firstseparate reaction zone and a second separate reaction zone which are inliquid-flow communication one with another via the body of liquid mediumbelow the lower extremity of the baffle means but out of gas flowcommunication one with another in gas spaces above a level of the bodyof liquid medium in each of the first and second separate reactionzones, a source of a first gas; first gas feed pipe means connected tosaid source of said first gas and extending downwardly in one of saidreaction zones for feeding said first gas from said source of said firstgas to a location in said first separation reaction zone submerged belowthe level of the body of liquid medium in said first separation reactionzone and above the lower extremity of the baffle means, first rotaryimpeller means located at a lower end of said first gas feed pipe meansand mounted for rotation about a verticals axis for distribution of thefirst gas in the body of liquid medium in the form of gas bubbles at thelocation in said first separation reaction zone, first shroud meanssurrounding said first rotary impeller means and having means defining aplurality of openings therethrough for permitting the gas bubbles of thefirst gas to pass through the first shroud means, a source of a secondgas independent of and different from said first gas; second gas feedpipe means independent of said first gas pipe feed means connected tosaid source of said second gas and extending downwardly in said secondseparation reaction zone for feeding said second gas different from thefirst gas from said source of said second gas to a location in saidsecond separation reaction zone submerged below the level of the body ofliquid medium in said second separation reaction zone and above thelower extremity of the baffle means, second rotary impeller meanslocated at a lower end of said second gas feed pipe means and mountedfor rotation about a vertical axis for distribution of the second gas inthe body of liquid medium at the location in the form of gas bubbles insaid other of said reaction zone, and second shroud means surroundingsaid second rotary impeller means and having means defining a pluralityof openings therethrough for permitting the gas bubbles of said secondgas to pass through the second shroud means.
 2. The apparatus of claim 1including first gas feed means communicating with said first gas ventmeans communicating with said first separation reaction zone above thelevel of the body of liquid medium in said vessel, and second gas ventmeans communicating with said second separation reaction zone above thelevel of the body of liquid medium in said vessel.
 3. The apparatus ofclaim 2 wherein said first and second rotary impeller means are mountedfor rotation about a vertical axis by an axle extending verticallydownwardly in the respective gas feed pipe means from exterior to thevessel.
 4. The apparatus of claim 1 including first gas vent meanscommunicating with said first separation reaction zone above the levelof the body of liquid medium in said vessel, and second gas vent meanscommunicating with said second separation reaction zone above the levelof the body of liquid medium in said vessel and also communicating withsaid first gas feed pipe means.
 5. The apparatus of claim 4 wherein saidfirst and second rotary impeller means are mounted for rotation about avertical axis by an axle extending vertically downwardly in therespective gas feed pipe means from exterior to the vessel.
 6. Theapparatus of claim 1 wherein said second rotary impeller means and saidsecond shroud means have smaller dimensions than that of said firstrotary impeller means and first shroud means.
 7. The apparatus of claim1 including means for removing insoluble product, said removing meanslocated adjacent to a surface of the liquid medium in each of saidreaction zones.
 8. The apparatus of claim 1 wherein said baffle meanscomprises a solid baffle element throughout the extension of said bafflefrom said upper closure to said lower extremity thereof.
 9. Theapparatus of claim 1 wherein said baffle means is solid above the levelof the body of liquid medium in said vessel is sufficiently porous belowthe liquid level to permit liquid flow therethrough but insufficientlyporous to permit gas flow therethrough.
 10. The apparatus of claim 1wherein said first and second shroud means are stationary.